The present invention relates to an arrangement and a method for improving manoeuvrability of a vehicle combination comprising at least three vehicle units interconnected by articulated joints. The arrangement and method is especially suited for heavy vehicle combinations comprising three or more vehicle units, where two vehicle units are provide with driven axles.
It is becoming more and more common to provide heavy vehicles with more than one driven axle. This is especially true for hybrid vehicles, where one axle is driven by a combustion engine in a regular fashion, and where a further driven axle is provided with an electrical machine. The electric machine can function as an electric motor for providing propulsive force to the vehicle when required, and can function as a alternator for retrieving energy when the vehicle brakes.
It is common to use only the electric motor to propel the vehicle in environmental sensitive areas, in city centres or when starting to drive. One situation in which the electric machine is used as an electric motor together with the combustion engine is when extra propulsive energy is needed, e.g. when the vehicle is accelerating. When the vehicle is propelled by both the combustion engine and the electric motor, it is important that the two wheel axles are synchronized in order to improve driveability and to reduce energy losses.
One area in which hybrid vehicles are used more and more is in city buses. The main purpose is to reduce emissions and to reduce energy cost. Even though most buses are rigid buses, it is also common to provide both articulated buses and bi-articulated buses, especially where a high capacity is required. A conventional articulated bus can be either of a pusher or a puller configuration. In a conventional pusher bus, only the rear C-axle is powered by a rear-mounted internal combustion engine, and the longitudinal stability of the vehicle is maintained by active hydraulics mounted under the articulated joint. In a conventional puller articulated buses, the engine is mounted either under the floor or off-center, at the side of the bus, and only the B-axle is powered.
When introducing a further driven wheel axle on a hybrid articulated bus, which comprises an electric machine, the further driven axle will be positioned in the vehicle unit that does not comprise the combustion engine. One obvious reason is that the only not powered axle, apart from the steering axle, will be positioned in the other vehicle unit. Another reason would be to improve the traction of the vehicle.
Thus, a pusher type hybrid articulated bus would comprise a rear axle, the C-axle, powered by a combustion engine and a rear axle of the front vehicle unit, the B-axle, powered by the electric machine. A puller type hybrid articulated bus would comprise a rear axle of the front vehicle unit, the B-axle, powered by a combustion engine and a rear axle, the C-axle, powered by an electric machine.
Normally, both axles will not be powered simultaneously. When the battery is low, when driving relatively fast or when driving outside of a city centre, the combustion engine will be used alone. When driving in environmental sensitive areas, in city centres or when the batteries are charged, only the electric motor is used to propel the vehicle. Only when a higher power is required, e.g. when a hill is to be climbed or when starting to drive, both the combustion engine and the electric motor can be used to propel the vehicle. When the vehicle is propelled by both the combustion engine and the electric motor, it is important that the two wheel axles are synchronized such that the longitudinal forces in the articulated joint are minimized.
Hybrid trucks are also becoming more common. In a smaller delivery truck having only one rear axle powered by a combustion engine, the electric machine will be connected to the same rear axle. These are referred to as parallel hybrid vehicles, since the two engines can work in parallel. It is also common to use serial hybrid vehicles, where the axle is only driven by the electric machine and where the combustion engine only drives an alternator.
For larger hybrid trucks and tractors, it is impractical to have a main rear axle which is powered by a combustion engine and an additional rear axle that is powered by an electric machine, since the additional axle is only used when necessary, i.e. when a heavy load is to be carried. Instead, it is suggested to provide the trailer with an electric machine to power an additional driven axle. This is especially advantageous for a tractor-trailer combination, since there is not enough room for extra batteries on a tractor and since a weight increase of a tractor could overload the front axle. By placing the complete hybrid system in the trailer would allow for a flexible solution without impairing the tractor properties. Also in such an arrangement, it is important that the two wheel axles are synchronized such that the longitudinal forces in the articulated joint are minimized.
It is also possible to place a hybrid system in a drawbar trailer which is to be connected to a truck, or in a trailer of a dolly-trailer combination which is also to be connected to a truck. By using both a combustion engine and an electric machine to propel the vehicle combination when extra power is required, an increased traction is obtained.
For all articulated vehicle combinations having a driven axle in more than one vehicle unit, it is important to synchronize the propulsive force to each wheel axle such that the different vehicle units propel the vehicle by an equal amount. If the drive axles are not synchronized properly, the vehicle combination may become unstable or energy may be lost. Such vehicles are thus provided with a control system that will synchronize the wheel axles.
An advantage of a long vehicle combination comprising several vehicle units is that it is in general more transport efficient since their load capacity is higher. An articulated bus may e.g. transport more passengers than a rigid bus.
One problem with a longer vehicle combination may be the stability of the vehicle combination. Even for vehicle combinations comprising two vehicle units, such as a tractor trailer combination, stability problems may arise when braking or turning. One stability problem that may arise is that the trailer starts swinging from side to side. This may happen when the vehicle combination travels with a relatively high speed and changes lane or drives in curves. The stability of the vehicle combination will normally correct itself when the vehicle travels straight, but this may still affect the traffic around the vehicle, either by bumping in to other vehicles or by scaring drivers in the vicinity. Another type of stability problem arises when the vehicle combination brakes. One such problem is known as jack-knifing, in which the trailer will spin around such that the tractor and trailer will resemble a folded pocket knife. This may happen when the trailer is braked less than the tractor. Another problem is known as swing out, where too much braking force by the trailer and low tyre/road friction may cause loss of lateral gripping force. This may cause the trailer to start swinging back and forth or to rotate.
There are several ways of improving the stability of a vehicle combination in order to avoid accidents. Solutions reducing the turning angle for the trailer have been proposed, unsuccessfully. Anti-lock brakes and electronic brake force distribution controlled by an electronic control unit has reduced some types of accidents. Such solutions are mostly designed for a vehicle combination having a single trailer, and use the brakes to stabilize the vehicle combination. For a vehicle combination provided with two driven axles and comprising more than two vehicle units, the proposed solutions will not suffice. There is thus still room for improvements.
It is desirable to provide an arrangement for improving manoeuvrability of a vehicle combination comprising three vehicle units interconnected by articulated joints, where two vehicle units are provided with a driven axle. It is also desirable to provide a method for improving the manoeuvrability of a vehicle combination comprising three vehicle units, where two vehicle units are provided with a driven axle.
In an arrangement according to an aspect of the present invention for improving manoeuvrability of a vehicle combination comprising a first vehicle unit, a second vehicle unit and a third vehicle unit interconnected by articulated joints, where the vehicle combination further comprises a distributed propulsion system, in which the vehicle combination comprises a first driven axle and a second driven axle and in which the first and the second driven axles can be controlled independently, means for determining the articulation angle between the vehicle units, means for determining a steering wheel angle of the vehicle combination, means for determining the speed of the vehicle combination, means for determining the yaw rate of the vehicle units and means for determining a delay value between the steering wheels of the vehicle combination and at least one articulated joint, the problem is solved in that the arrangement is adapted to control a desired articulation angle between of the first and the second articulated joints by coordinating the force ratio between the first driven axle and the second driven axle by using the determined yaw rate or articulation angle of the vehicle units and the determined delay value.
By this first embodiment of the arrangement, the arrangement will control a desired articulation angle of the articulated joints by coordinating the force ratio between the first driven axle and the second driven axle. In this way, the rotational torque acting on the articulated joints can be increased or decreased, which in turn will affect the actual articulation angles.
In one example, the articulation angle of the first and the second articulated joints is the same. This may be the case when the different vehicle units have substantially the same length. With the same articulation angles, the control of the vehicle combination is simplified and the manoeuvrability of the vehicle combination is increased. When the vehicle combination turns, the behaviour of the two articulated joints will be similar and symmetrical. There is thus no need to introduce synchronization means between the two articulated joints. A certain damping in each articulated joint may be of advantage.
In another example, the articulation angle of the first and the second articulated joints will differ. This will be the case when the vehicle units have different lengths or when the articulated joints differ from each other. With different articulation angles, a mean value for the first and the second articulation angle can be used as a single articulation angle.
In one example of the invention, the first driven axle is provided in the first vehicle unit and the second driven axle is provided in the third vehicle unit. An example of such a vehicle combination is a bi-articulated bus having a combustion engine mounted in the rear part of the bus, and an electric motor mounted in the front part of the bus. By adding an electric driveline to the front part of the bus, a low floor of the bus can be preserved since both an electric motor and batteries can be fitted below the front low floor of a standard bus. In some bi-articulated buses, the combustion engine is mounted in the front part of the bus. In such a bus, it is possible to mount an electric driveline in the rear part of the bus, and still preserve the low floor of the rear part.
The vehicle combination may also be e.g. a truck with a dolly and a trailer. In such a vehicle, the combustion engine is mounted in the truck and the trailer will be provided with an electric driveline, where an electric motor and batteries are mounted in the trailer. The vehicle combination may also be a truck with a drawbar trailer, where the trailer is provided with a hybrid electric driveline.
By increasing the propulsive force of the rearmost driven axle in relation to the front driven axle, the rear vehicle unit will push against the articulated joints which in turn will produce a positive rotational torque on the articulated joints. The articulation angle will in this case increase and the radius of the travelled path will decrease. This function can be used when the articulated vehicle combination is travelling through a sharp bend, in order to decrease the turning radius of the vehicle combination and to allow the vehicle combination to follow the intended path.
By decreasing the propulsive force of the rearmost driven axle in relation to the front driven axle, the rear vehicle unit will apply a pulling force on the articulated joints which in turn will produce a negative rotational torque on the articulated joints. The articulation angle will in this case decrease and the radius of the travelled path will increase. This function can be used when the articulated vehicle combination is to be straightened out, in order to stabilize the vehicle combination.
By varying the force ratio between the first driven axle and the second driven axle, it is possible to create either a pushing or compressive force or a pulling or stretching force on the articulated joints. In this way, the actual articulation angle of the articulated joints can be controlled to follow a desired articulation angle. The desired articulation angle may e.g. represent a desired path that the vehicle combination is following, and may be based on the steering wheel angle of the leading vehicle unit and the determined delay value. A desired path may also be determined from a vehicle model having the articulation angle, the steering wheel angle and the vehicle speed as input. The determined delay value is a delay time based on the speed of the vehicle combination and corresponds to the distance between the front axle and respective articulated joint. If the vehicle travels with a relatively low speed or stands still, it is preferred to let the delay value be a distance instead. In this way, the control system can start to control the desired articulation angles when the vehicle combination starts to travel, based on the measured articulation angles and the distance.
In another example of the invention, the first driven axle is provided in the second vehicle unit and the second driven axle is provided in the third vehicle unit. An example of such a vehicle combination is a bi-articulated bus having a combustion engine mounted in the rear part of the bus, and an electric motor mounted in the middle part of the bus. By adding an electric driveline to the middle part of the bus, a low floor of the bus can be preserved since both an electric motor and batteries can be fitted below the middle low floor of a standard bus.
The inventive arrangement can be used to improve the manoeuvrability of the vehicle combination and to stabilize the vehicle. One condition in which it is advantageous to improve the manoeuvrability of the vehicle combination is when the vehicle combination is to travel through a sharp turn. Such a manoeuvre is mostly done when the vehicle combination travels at a low speed. It is thus of importance to only allow such a manoeuvre when the actual vehicle speed is below a predetermined vehicle speed. Such a predetermined speed may be below 15-30 km/h.
One condition in which it is advantageous to improve the stability of the vehicle combination is when the vehicle combination travels straight or along a slightly curved path at a relatively high speed. In such a condition, it may be of advantage to create a slight stretching force on the articulated joint which will counteract any longitudinal instability, caused e.g. by a side wind or uneven road surface. It may also be of advantage to apply a slight stretching force on the articulated joint when the vehicle is travelling on a slippery road, in order to prevent jack-knifing.
The inventive arrangement is suitable for different vehicle combinations. Such vehicle combinations include vehicle combinations having two or more articulated joints, such as a tractor, a dolly and a semi-trailer or a bi-articulated bus.
In a method for method for improving the manoeuvrability of a vehicle combination comprising a first vehicle unit, a second vehicle unit and a third vehicle unit, where the vehicle combination comprises a first driven axle and a second driven axle, where the vehicle units are interconnected by articulated joints, the steps of determining the articulation angles of the articulated joints, determining a steering wheel angle of the vehicle combination, determining the speed of the vehicle combination, determining the yaw rate of the vehicle units, determining a delay value between the steering wheels of the vehicle combination and at least one the articulated joint, and controlling a desired articulation angle between the vehicle units by coordinating the force ratio between the first driven axle and the second driven axle by using the determined yaw rate of the vehicle units and the determined delay value are comprised.
With the inventive method, the manoeuvrability of a vehicle combination comprising at least three vehicle units interconnected by articulated joints can be improved.